A time of flight (TOF) three dimensional (3D) camera acquires distances to features in a scene that the TOF-3D camera images by determining how long it takes temporally modulated light that the camera transmits to illuminate the scene to make a “round trip” from the camera to the features and back to the camera. The known speed of light and a round trip time to a given feature in the scene determined by the TOF-3D camera are used to determine a distance of the given feature from the TOF-3D camera. A “range image” of the scene generally refers to the distances acquired by the TOF-3D camera to substantially all features in the scene that the TOF-3D camera images at substantially a same given time.
In a “gated” TOF-3D camera, a train of light pulses may be transmitted by a light source to illuminate a scene that the camera images. Upon lapse of a predetermined, same delay interval, hereinafter an “exposure delay”, after each light pulse in the train of light pulses is transmitted, the camera is shuttered, or “gated”, ON, for a short exposure period, which ends when the camera is shuttered, or “gated”, OFF. The camera images light from the transmitted light pulses reflected by features in the scene that reaches the camera during the exposure periods on pixels of a photosensor that the camera comprises. The light reflected by a feature in the scene from a transmitted light pulse reaches a pixel on which the feature is imaged as a reflected light pulse having pulse shape substantially the same as the pulse shape of the transmitted light pulse from which it was reflected.
An amount of reflected light from a feature imaged on a pixel that is registered by the pixel during an exposure period, is a function of a correlation between the pulse shape of the reflected light pulses and an exposure period sensitivity profile of the pixel for registering light during the exposure periods. The correlation, which may also be referred to as a correlation between the light pulse and exposure period, is a function of the round trip time for light to propagate to the feature and back to the gated TOF-3D camera and the known exposure delays. The amounts of light in the reflected light pulses registered by the pixel during the exposure periods associated with the transmitted train of light pulses, and the known exposure delay, are used to determine a round trip time of light to and back from the feature and therefrom distance to the feature.
A pixel registers incident light by accumulating positive or negative electric charge, hereinafter also referred to as “photocharge” provided by electron-hole pairs generated by photons in the incident light. Circuitry in the TOF-3D camera converts photocharge accumulated by the pixels into voltages that are used as measures of the amounts of photocharge they respectively accumulate. An amount of light that a pixel registers may refer to the amount of photocharge accumulated by a pixel responsive to incident light, or to a voltage generated responsive to the accumulated photocharge. A response of a pixel refers to an amount of light that the pixel registers responsive to incident light.
Pulse shape of a light pulse refers to a shape of a curve representing intensity of light on a surface through which the light pulse passes as a function of time, normalized to a maximum intensity of light in the light pulse. Pulse shape may also refer to the shape of a curve representing the normalized intensity as a function of position along the width of the light pulse. Intensity of light in the light pulse at a given time, or location along the light pulse width, is equal to a product of a magnitude of the pulse shape at the time or position times a maximum intensity of the light pulse. An exposure period “sensitivity profile”, or exposure period “profile”, refers to shape of a curve representing sensitivity of a pixel in the TOF-3D camera photosensor for registering light between ON and OFF times of the exposure period. Sensitivity of a pixel for registering light refers to an amount of photocharge that the pixel registers per unit of optical energy incident on the pixel.
Whereas an amount of light registered by a pixel during the exposure periods is a function of the correlation between the reflected light pulses and the exposure periods, the amount is also a function of intensity of the reflected light pulse and thereby of reflectivity of the feature. Reflectivity of the feature is generally not known from the registered light. Furthermore, during an exposure period a pixel registers background light from a scene and light from “multipath light”, in addition to reflected light from the feature. Background light is not a function of round trip time. Multipath light refers to light in reflected light pulses that has undergone more than one reflection in reaching the TOF-3D camera rather than being reflected once, directly back to the camera from a feature imaged by the camera. Multipath light reaches the TOF-3D camera delayed relative to directly reflected light. An amount of light that the pixel registers during an exposure period is therefore affected by variables other than round trip time that vitiate distance information in the amount of registered light.